Code style, inconsistent APIs, a feature that's barely tested by its own contributor, introducing dependencies on innards that were planned to be removed by an ongoing refactoring project, etc. So many ways contributions can go wrong.
The entire context of this thread is that adding committers can introduce risk in the quality of accepted contributions. Building a trusted network of gatekeepers is hard.
"Style" is more than indentation - my favourite example is something I worked with very closely, but always needed a cheat-sheet on my desk - the PHP arrays api.
Just look at the docs for array_search and array_replace.
This is much worse in managed memory languages - do you allocate memory, who frees it, who can use a buffer in zero-copy mode, who can't ... whether the functions are always re-entrant.
All that said - any originating coder who burns out due to support issues, killing the community is measurably worse off than someone who has thousands of users who demand that you support such "legacy" APIs for another six months.
I'm talking here about a developer API that is extremely flexible and extensible. Even one-liner contributions can have bad effects, and virtually all contributions I receive "work" at a superficial level and if they provide tests the tests usually pass. "made it through testing" is a low bar, the bigger implications of a patch need to be considered.
Examples of PRs that "work" exactly as intended yet are the wrong way to solve the problem:
if I had released that fully and the whole world coded to "mysql.insert.values.bar == 5", that's broken API since it breaks the values() method. Backwards-incompatible fix required.